The present invention relates to an electric power supply unit for plasma systems such as plasma-processing or coating devices wherein electric arcs or arcing may occur, particularly between the electrodes, in accordance with the introductory clauses of the independent Patent Claims.
Plasma systems for which the electric power supply units of the claimed general type are intended are suitable for a great number of potential applications and are employed, for instance, for target sputtering. As a rule, they produce an electrical power ranging from roughly a few kW up to more than 100 kW. In typical cases, the operating voltages applied to the electrodes is in the range of 400 VV. Variations in the upward or downward directionxe2x80x94also as a function of the respective applicationxe2x80x94are, of course, possible.
Specifically in so-called xe2x80x9creactivexe2x80x9d sputtering, a technique applied, for instance, for the production of oxide or nitride films, or in the application of reactive gases problems occur as a result of disruptive breakdown and/or the formation of insulating layers on the conducting target, with the consequence that parasitic capacitors may be formed.
The most different passive and active circuits have become known which serve to extinguish electric arcs that have been formed as a result of disruptive breakdown. All of these circuits share the common feature that the voltage applied to the electrode for a defined period is disconnected. Reference is made here to the U.S. Pat. Nos. 4,692,230, 5,009,764, 5,015,493 and 5,682,067 just as examples, which are, by the way, also explicitly referred to for an explanation of all particulars and potential applications not described here in more details.
The U.S. Pat. No. 5,682,067 discloses, by the way, that state of prior art that the wording of the introductory clauses of the independent Patent Claims started out from.
The disadvantage of the circuits known from these prior art documents is the fact that after re-activation of the operating voltage the system requires a comparatively long time until it is capable again xe2x80x9cto sputter with full powerxe2x80x9d or to process the target in any other manner.
Analogous findings apply when the system is operated in a so-called asymmetric pulsed operation for preventing insulating layers from forming on the target.
It is moreover known to disconnect the voltage applied to the electrodesxe2x80x94independently of whether disruptive breakdown or arcing occursxe2x80x94periodically for a short interval. In such an approach the duty cycle must be so selected that during the period in which the voltage is applied disruptive breakdown or an electric arc will not occur. Even though this provision is suitable to prevent the occurrence of disruptive breakdown or electric arcs the periodic disconnection and subsequent re-application of the voltage reduces, however, the efficiency of the system substantially.
The present invention is based on the problem of providing an electric supply unit for plasma systems, wherein, after re-application of the operating voltage to the electrodes, the system reaches its full power as quickly as possible, and/or wherein the reduction of efficiency is as low as possible, which is due to the occurrence of electric arcs or disruptive breakdown, e.g. as a result of the xe2x80x9cprophylacticxe2x80x9d disconnection of the operating voltage.
Inventive solutions to this problem are defined in the Patent claims 1 or 18, respectively. Improvements of the invention are the subject matters of the dependent Claims.
In accordance with the invention the application starts out from an electric power supply unit comprising a source of d.c. voltage or direct current, respectively, whose output terminals are connected via at least one inductive resistor and a power switch to the electrodes of the plasma system, and possibly a circuit for detecting electric arcs or disruptive breakdown, which, upon occurrence of an electric arc or disruptive breakdown, operates the switch in such a way that electric energy is no longer applied to the electrodes.
Starting out from such a known supply unit, the invention consists in the aspects that the inductive resistor(s) is (are) connected to a respective recovery diode and that the switch is a series switch.
Based on this configuration, the inventive supply unit operates as follows:
The switch, that switches the electric power applied to the electrodes (hereinafter also referred to as power switch), enables the current conduction between the d.c. voltage source and the electrodes upon detection of an electric arc. As soon as the power switch is opened the diodes, which are connected in the disabling direction relative to the normal operating voltage, prevent an increase of the voltages at the inductive resistors, producing the simultaneous effect that the current in the inductive resistors will decrease only very slowly. When the power switch is energised again after a short period alreadyxe2x80x94roughly a few milliseconds are typicalxe2x80x94the current stored in the inductive resistors is available immediately so that the system reaches its full performance more rapidly than this is the case in prior art. The inventive power supply unit hence presents a power source characteristic when the power switch is switched on again.
Hence the power losses, which occur in the inventive electric supply unit as a result of the disconnection of the operating voltage for extinction of electric arcs, are substantially smaller than those in prior art.
This inventive basic circuitxe2x80x94as defined in claim 1xe2x80x94may be extended in consideration of various aspects which may also be combined with each other:
It is possible in particular to provide an inductive resistor with an associated recovery diode in each line connecting a terminal of the d.c. voltage or direct-current source, respectively, with the respectively associated electrode. As a result, a particularly symmetrical circuit structure is achieved. It is also possible, however, to use only one inductive resistor; in such a case only one recovery diode is required.
The most different power switches can, of course, be used as switches. In view of the high performance, however, the switch is preferably an IGBT element.
Moreover, the most different power supply units may be used as mains supply circuits; equally in view of the high performance it is preferred, however, that the d.c. voltage source is a switching-controller supply unit.
In such a case it is expedient to connect a capacitor between the terminals of the d.c. voltage or direct-current source. This capacitor serves to avoid voltage peaks possibly occurring as a result of the inductive resistors in the feeders in the supply unit. Moreover, this capacitor is charged to the full no-load voltage level in the period in which the power switch is de-energised so that when the power switch is switched through the full voltages are available for exciting the plasma.
The inventive supply unit can be employed for the most different plasma systems such as plasma installations where the first electrode comprises at least one cathode disposed in a plasma chamber while the other electrode is the housing of the plasma chamber. The inventive supply unit presents furthermore the special advantage that it is also suitable for use with plasma systems wherein at least two cathodes are provided separately from each other, whereof each is connected via a series switch to an inductive resistor on the first terminal of the d.c. voltage source, that presents a recovery diode.
In such systems an alternating change-over is possible, for instance, between the cathodes. This provision entails the advantage that electric arcs being xe2x80x9cin the incipient phasexe2x80x9d will be reliably extinguished in the comparatively long period during which the change-over to the other cathode is performed. It is, of course, also possible, however, to perform sputtering from several cathodes.
In an improvement of the invention a diodes is connected between the electrode terminals or the pairs of electrodes, respectively. The function of this diode will be explained in the following:
As soon as the power switch is opened the voltage on the inductive feeder resistors commutates to the plasma chamber so that voltage peaks would occur without any further circuitry provisions. The diode connected between the electrode terminals shorts this voltage so that the energy stored in the inductive feeder resistors will be reduced.
It is furthermore possible to connect an LC circuit in parallel with the diode, which, as a passive element, supports the extinction of the electric arc.
It is particularly preferable, however, to support the extinction of the electric arc by active circuit elements:
To this end a capacitor may be connected in series with the diode. Then another series switch is connected between the cathode terminal and the diode/capacitor connection point. With this circuitry layout the energy that derives from the line inductance and the current conduction is re-charged as voltage into the capacitor when the power switch is opened. After a short delay, this voltage is applied via the further series switch to the electrodes so that the electric arc will be extinguished substantially more rapidly than this would be the case in a passive circuit.
To achieve an inverted voltage of appropriate magnitude for extinction even with small line inductance levels or small currents it is furthermore possible to provide an inverted-voltage source.
This inverted-voltage source may be an (auxiliary) d.c. voltage source whose negative terminal is connected to the anode and whose positive terminal is connected to the first terminal of the further series switch via a diode connected in the forward direction.
In any case it is preferable that the circuit for electric arc detection controls not only the power switch but also the additional switch.
The circuit for electric arc detection is capable of evaluating at least one of the following criteria:
voltage collapse,
maximum voltage limit is exceeded,
voltage drops below the minimum voltage limit,
rapid current increase,
maximum current limit is exceeded.
Moreover, a controller may be provided in a manner known per se, which may be a constituent of the circuit for detecting electric arcs in particular and which, for xe2x80x9cprophylaxisxe2x80x9d, disconnectsxe2x80x94and preferably commutesxe2x80x94the voltage applied to the electrodes with a fixed period. When the voltage applied to the electrodes is regularly commuted for a brief interval the occurrence of electric arcs can be substantially reduced or can electric arcs be extinguished as early as in their incipient phase.
Due to the inventive configuration, that results in rapid achievement of the power after re-connection, the regular commutation of the voltage leads only to a very slight reduction of the sputtering performed, provided that the interval during which the voltage applied to the electrodes is commuted is substantially shorter than the interval during which plasma operation is performed. Moreover, the period during which the voltage is commuted must, of course, also be so short that the current will be stored in the xe2x80x9cinductive resistor/recovery diodexe2x80x9d circuit.
The frequency at which the controller disconnects or commutates the voltage applied to the electrodes may be as high as up to 100 kHz.
Instead of periodic prophylactic commutation with an invariable period that is independent of the process parameters and the frequency of electric arcs or disruptive breakdown as a function of the latter, it is preferable, however, that the controller, in the manner of an adaptive controller, sets the voltage connection period in such a way that an electric arc or disruptive breakdown does xe2x80x9cjustxe2x80x9d not occur. In this further solution to the problem underlying the present invention, which may also be employed in addition to the solution according to Claim 1, it is expedient to provide a controller or closed-loop controller that in response to the occurrence of an electric arc or disruptive breakdown extinguishes this arc by disconnecting the voltage applied to the electrodes or by change-over to an inverted voltage for a defined period (deactivation interval), and that the controller or closed-loop controller, respectively, reduces the activation interval of the voltage causing the plasma operation upon occurrence of at least one electric arc or disruptive breakdown.
In this configuration the special advantage is entailed that when the controller or closed-loop controller reduces the activation interval to a period shorter than the actual interval between two electric arcs or events of disruptive breakdown occurring in succession. As the operating parameters of a system may vary during a processing operation it is furthermore preferred that the controller or closed-loop controller also prolongs the activation interval:
To this end it is possible, for instance, that the controller or closed-loop controller prolongs the activation interval when an electric arc or disruptive breakdown did not occur or only a defined number of electric arcs or events of disruptive breakdown has occurred over a defined period or over a defined number of activation intervals. The controller and closed-loop controller may then prolong the activation interval up to continuous operation.
In an approach to avoid excessive weighting of the random occurrence of arcs in adaptive control, it is moreover preferable that the controller or closed-loop controller, respectively, shortens the activation interval only when during a defined period a certain number of electric arcs or events of disruptive breakdown has occurred.
It is moreover preferable that the controller or closed-loop controller, respectively, connects the voltage, that is applied to the electrodes at the outset of the plasma operation, either permanently or for a defined activation interval (that can be predetermined prior to the commencement of the process), then disconnects or preferably commutates it for a defined deactivation interval, and then connects the voltage again, and reduces the activation interval of the voltage upon occurrence of electric arcs or disruptive breakdown.
In order to be able to match the inventive supply unit to the most different processing or coating processes it is moreover expedient that the initial values of the activation interval and the deactivation interval and/or the number of electric arcs or disruptive breakdown events as well as the unit of time during which this number occurs can be adjusted.
It is moreover expedient in any case that the activation interval is linked up with the frequency of occurring electric arcs or disruptive breakdown via a variable characteristic that may be dependent on further process parameters.
To keep the reduction of the power of the plasma system as low as possible it is furthermore advantageous that the controller or closed-loop controller, respectively, optimises the disconnection or commutation interval, respectively. The disconnection or commutation interval is set to be xe2x80x9cjustxe2x80x9d so long that the electric arc or disruptive breakdown will be extinguished xe2x80x9cwith a lasting effectxe2x80x9d.